U.S. patent number 5,329,259 [Application Number 07/986,823] was granted by the patent office on 1994-07-12 for efficient amplitude/phase modulation amplifier.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Charles A. Backof, Robert E. Stengel.
United States Patent |
5,329,259 |
Stengel , et al. |
July 12, 1994 |
Efficient amplitude/phase modulation amplifier
Abstract
A high efficiency amplitude/phase modulation amplifier circuit
(100) includes a first (102) and a second (106) high efficiency
amplifier. These amplifiers (102 and 106) amplify two constant
amplitude/phase modulated signals. A combiner (104) combines the
output signals from the amplifiers (102) and (106) to produce a
combined signal to a load (108). Two shunt elements (202 and 204)
are included to prevent the reactive components of the combined
signal from reaching the amplifiers (102) and (106). With no
reactive components reflected back, the amplifiers (102 and 106)
can remain non-linear even though they are used to amplify an
amplitude/phase modulated signal which includes Amplitude
Modulation (AM) components.
Inventors: |
Stengel; Robert E. (Ft.
Lauderdale, FL), Backof; Charles A. (Coral Springs, FL) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
|
Family
ID: |
25532781 |
Appl.
No.: |
07/986,823 |
Filed: |
February 11, 1993 |
Current U.S.
Class: |
332/103;
332/145 |
Current CPC
Class: |
H03F
3/602 (20130101); H04L 27/362 (20130101); H03F
3/2176 (20130101) |
Current International
Class: |
H03F
3/60 (20060101); H04L 27/34 (20060101); H04L
27/36 (20060101); H04L 027/36 () |
Field of
Search: |
;332/103,104,105,145,151
;330/295,124R,10 ;375/39,42,67 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Grimm; Siegfried H.
Attorney, Agent or Firm: Ghomeshi; Mansour M.
Claims
What is claimed is:
1. An amplitude/phase modulation amplifier circuit coupled to a
load, comprising:
first amplifier for amplifying a first phase modulated signal to
produce a first amplified signal;
second amplifier for amplifying a second phase modulated signal to
produce a second amplified signal;
a combiner for combining the first and the second amplified signals
with minimum reflection to produce a high efficiency amplified
amplitude/phase modulated signal; and
an isolator coupled to the combiner for maximizing efficiency.
2. The amplifier circuit of claim 1, wherein the first amplifier
includes a constant output amplifier.
3. The amplifier circuit of claim 1, wherein the combiner includes
an isolator for maximizing efficiency.
4. The amplifier circuit of claim 1, wherein the combiner includes
a summer.
5. The amplifier circuit of claim 1, wherein the second amplifier
includes a constant output amplifier.
6. A high efficiency amplitude/phase modulation amplifier circuit,
comprising:
a first amplifier for amplifying a first phase modulated signal,
the first phase modulated signal having a fixed amplitude, a
frequency, a fixed phase component, and a first variable phase
component;
a second amplifier for amplifying a second phase modulated signal,
the second phase modulated signal having the fixed amplitude, the
frequency, and the fixed phase component of the first phase
modulated signal along with a second variable phase component;
a combiner for combining the first and the second phase modulated
signals to produce an amplified amplitude/phase modulated signal
having a reactive component, the combiner having means for
preventing the reactive component of the amplified amplitude/phase
modulated signal from loading the first or the second amplifier;
and
an isolator for accommodating a variable load to appear fixed to
the combiner in order to maximize the efficiency of the amplifier
circuit.
7. The high efficiency amplifier circuit of claim 6, wherein the
combiner includes a plurality of transformers.
8. The high efficiency amplifier circuit of claim 7, wherein the
combiner includes an isolator for accommodating a variable load to
appear fixed to the plurality of transformers in order to maximize
the efficiency of the amplifier circuit.
9. The high efficiency amplifier circuit of claim 6, wherein the
first and second amplifiers include class E amplifiers.
10. The high efficiency amplifier circuit of claim 6, wherein the
first and second amplifiers include switching amplifiers.
11. A Quadrature Amplitude Modulation (QAM) transmitter,
comprising:
an oscillator for generating a carrier signal;
a modulator for modulating the carrier signal to produce a first
and a second phase modulated signal:
a QAM amplifier circuit, the amplifier circuit comprising:
first amplifier for amplifying the first phase modulated signal to
produce a first amplified signal;
second amplifier for amplifying the second phase modulated signal
to produce a second amplified signal;
a combiner for combining the first and the second amplified signals
to produce an amplified QAM signal having a reactive component, the
combiner having means for minimizing the reflection of the reactive
component onto the first and second amplifiers in order to optimize
efficiency; and
an isolator coupled to the combiner.
12. The transmitter of claim 11, further comprising an antenna
coupled to the QAM amplifier for transmitting the amplified QAM
signal.
13. The transmitter of claim 11, wherein the first and second
amplifiers include class E amplifiers.
Description
FIELD OF THE INVENTION
This invention is related in general to modulators and more
specifically to quadrature amplitude modulators.
BACKGROUND OF THE INVENTION
Amplitude/phase modulation such as Quadrature amplitude modulation
(QAM) combine amplitude and phase modulation to produce a higher
information throughput for a given spectral bandwidth than is
available by using phase or amplitude modulation alone. The
spectrum efficiency is however gained through the detriment of
power efficiency. Although the phase modulation in a QAM signal
takes advantage of the high DC to RF power efficiency, the
efficiency of the QAM signal is limited to that of the amplitude
modulation amplifiers. As is known, amplitude modulation requires
the use of linear power amplifier systems to preserve the
information modulation in the carrier envelope. Linear amplifiers
are notorious for their efficiency. For example, a single ended
linear power amplifier (class AB) will have a maximum efficiency at
a peak power of 37%. This efficiency decreases as the output power
level decreases with the average efficiency being lower than the
peak power efficiency. It can be seen that although QAM signals
provide an improvement in spectral efficiency they are limited to
the linear power amplifier efficiency of AM systems. It is
therefore appreciated that a high efficiency QAM system is highly
desired that would not suffer from the low efficiency of amplitude
modulation linear amplifiers.
SUMMARY OF THE INVENTION
Briefly, in accordance with the present invention, a high
efficiency amplitude/phase modulation amplifier circuit is
disclosed. The amplifier circuit includes a first and a second
amplifier for amplifying a first and a second phase modulated
signal to produce a first and a second amplified signal,
respectively. A combiner is used to combine the first and the
second amplified signals to produce an amplified amplitude/phase
modulated signal. The reactive component of the amplified signal is
prevented from loading the first and second amplifier, hence
rendering the circuit highly efficient.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an amplifier circuit in accordance
with the present invention.
FIG. 2 is a block diagram of the components of a combiner in
accordance with the present invention.
FIG. 3 is a plot of the load variation of the amplifier of FIG. 1
in accordance with the present invention.
FIG. 4 shows a simulated plot of the efficiency performance of the
amplifier in accordance with the present invention.
FIG. 5 is a block diagram of a communication device in accordance
with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a block diagram of an amplifier circuit 100 in
accordance with the present invention is shown. The amplifier
circuit 100 includes amplifiers 102 and 106. These amplifiers may
be any of well known constant output high efficiency amplifiers;
such as class C or E. The input to amplifier 102 is:
The input to amplifier 106 is:
As can be seen, the two inputs have the same amplitude A(t) and
different time varying phase components q.sub.1 (t) and q.sub.2
(t). Each of these two phase components is carrying valuable
information. The amplitude A(t), however, does not carry any
information. Indeed, constant output high efficiency amplifiers may
be used in amplifiers 102 and 106 because of lack of information
carried by the amplitudes of the input signals. Amplified signals
at the output of amplifiers 102 and 106 are coupled to a combiner
104. The output of the combiner 104 is then coupled to a load 108.
In the preferred embodiment, the combiner 104 is a transmission
line impedance transformer and the load 108 is an antenna. The
combiner 104 may be a summer that sums the two output signals of
amplifiers 102 and 106 to produce an amplitude/phase modulated
signal. In the preferred embodiment the signal produced is a QAM
signal. It is understood that similar circuits may be used to
produce a Single Side Band signal (SSB) or a high efficiency
Amplitude Modulation (AM) signal.
FIG. 2 shows a block diagram of the components of the combiner 104
in accordance with the present invention. The inputs are shunted
with two reactive components 202 and 204. Two transformers 206 and
208 couple the inputs to an isolator 210. The output of the
isolator 210 is shown coupled to the load 108. It is known in the
art that loads; such as antennas, vary in impedence to some degree
despite a desire otherwise. The variablity of the load 108 is
handled via the isolator 210. The isolator 210 accomodates for some
variation in the load 108 without allowing this variation to
adversely affect the operation of the amplifier 100. In other
words, the isolator 210 accommodates the variable load 108 to
appear fixed to transformers 206 and 208. The isolator 210 provides
the isolator means of the present invention and may be any of well
known non-reciprocal impedance isolators, such as circulators.
It is noted that although the load 108 appears constant to the
transformers 206 and 208 it is desired that it appear variable to
the output of the amplifiers 102 and 106. The combiner 104 in
combination with the phase relationship of the signals at the
output of amplifiers 102 and 106 provide for a variable impedance
to appear at the output of the amplifiers 102 and 106.
As stated, since the amplitude A(t)in signals 110 and 112 does not
carry any information the amplifiers 102 and 106 may be of the
non-linear high efficiency type. In the preferred embodiment two
high efficiency class E amplifiers are used.
The output of the combiner 104 is
and using trigonometric identifies and replacing q.sub.1 and
q.sub.2 with the relations in below, the following results
where
For the purpose of our calculation it is assumed that
Note that B(t) and .theta.(t) are the amplitude and phase
modulation of the output signal. Now fix the amplitude of the two
combined signal sources A(t) as a constant in time at A. The output
signal can be amplitude modulated with a phase modulation +.PHI.(t)
and -.PHI.(t) on the combined fixed amplitude signals. In addition,
the output can have a desired phase and amplitude modulation by
adding an offsetting phase modulation .PHI.(t) about the desired
.theta.(t) modulation. The result is an amplitude and phase
modulation signal generated from combining two constant amplitude
phase modulated only signals.
With respect to the load R.sub.o and the desired phase modulation
.theta.(t) the currents i.sub.1 and i.sub.2 from FIG. 2 can be
expressed in Cartesian form as: ##EQU1## applying the desired
output voltage B(t) Z.sub.1 and Z.sub.2 may be determined as
follows ##EQU2## The impedances Z.sub.1 and Z.sub.2 are transformed
by the characteristic impedance of the quarter wave transmission
lines 206 and 208 and the following relation with a 180 degree
phase shift. ##EQU3## where Z.sub.o is the characteristic impedance
of transformers 206 and 208. Using this relation and equation (1)
with B(t).sub.max =2A(t), and normalizing to get the following:
##EQU4##
FIG. 3 shows a plot of the above function with normalized magnitude
402, and real and imaginary values 404 and 406, respectively. This
equation can be expressed in ratio terms of desired output voltage
(Vr) or phase offset .PHI.(t) using equation (1) with
B(t).sub.Max=2A (t). ##EQU5##
Using this and the expected range of output power, a shunt reactive
value can be determined for compensating the combined reactance
applied back to the power amplifier. FIG. 3 shows the added
reactance of XL=1/B.sub.L to offset the reactance as a result of
the combined phase offset signals. This reactance could be a fixed
value optimized for the expected output power range of operation,
or a variable element as a function of the phase offset.
The two shunt elements 202 and 204 stop the reflection of the
reactive components of the combined signal to reflect back to the
amplifiers 102 and 106. In other words, the shunt elements 202 and
204 stop the dissipation of power in the reactive element that
results when the two amplified signals are combined with a phase
offset other than 0.degree. or 180.degree.. The minimum power
dissipated in this reactive element results in the high efficiency
operation of the circuit 100. It is understood that the two shunt
elements 202 and 204 may take any one of several topologies. One
such topology is an element between the two inputs of the
transformers 206 and 208. Although the two shunt elements 202 and
204 optimize the efficiency of the combined output signal at a
particular power level, it is understood that their values could be
altered as a function of the phase offset to provide efficiency
optimization at various output power levels. Using the normalized
graphs of FIG. 3 one could determine the value of the shunt
elements 202 and 204 at different power levels.
Now to get P.sub.output /P.sub.output Max we must obtain the
desired V.sub.Lx or I.sub.Lx and process them with Z.sub.Lx for the
given amplifier class considered.
It is known that ##EQU6## using the impedance transformer relation
with the assumption of a lossless element, the following
results,
and
applying equations 2, 4, and 5 into equation 3, the following
results:
Solving for P.sub.output Max ##EQU7## If the reactance elements 202
and 204 have infinite reactance or zero susceptance
(Im(Y.sub.Lx)=B.sub.Lx =0) then the above equation is reduced to
##EQU8##
These results indicate that the output of the amplifier 100 is
composed of two components; a real power delivered to the
resistance of the load 108 and a reactive power as a function of
the amplifier phase offset. This reactive power will result in an
efficiency reduction if reactive shunt elements 202 and 204 are not
used. An appreciable efficiency improvement is realized in the
amplifier 100 using the two reactive shunt elements 202 and
204.
Some class of amplifiers, such as class AB, when used as amplifiers
102 and 106 take special advantage of the characteristics of the
combiner 104. In fact, the combiner 104 can be thought of rendering
the load 108 variable to the amplifiers 102 and 106 for these
classes of amplifiers. In other words, the load to which the
amplifiers 102 and 106 source current is continuously changing via
the combiner 104. The change in the load characteristic allows the
amplifiers 102 and 106 to operate at their peak power with maximum
efficiency.
FIG. 4 shows a plot 502 of collector modulation of class E power
amplifiers with a normalized shunt reactance B.sub.L =0.08.OMEGA..
Also shown in FIG. 4 are plots 504, 506, 508, and 510. Plot 504
graphs the collector modulation of the class E power amplifiers
without the shunt elements. Plots 506, 508, and 510 show the
performance of linear class B, AB, and A power amplifiers,
respectively. For linear amplifiers the efficiency is directly
proportional to the output power level, therefore the average
efficiency corresponds to the efficiency at the average output
power level. For the non-linear phase combined class E power
amplifier the power output efficiency relation is not proportional.
This means that the average efficiency for the class E power
amplifier is determined by applying the output power level
distribution function against the efficiency output power level
curve. The result may be an average efficiency that is different
from the average power level efficiency. As can be seen the
efficiency of the class E amplifier with the shunt elements is
significantly better than the one without the shunt elements.
In summary, two phase modulated signals are amplified and combined
to produce a QAM signal. By shifting the amplification to the front
end and part of the modulation to the back we have achieved high
efficiency amplification of an amplitude modulated signal. The
amplifiers 102 and 106 do not have to be low efficiency linear
amplifiers, as is required for amplitude modulated signals. In
essence, the two step modulation is accomplished at two distinct
points in the amplifier chain. The information is initially placed
in the phase of two distinct signals to allow their non-linear
amplification. Once amplified, the two signals are combined to
produce a QAM signal. In effect, this technique allows the use of
high efficiency non-linear amplifiers to amplify an amplitude
modulated signal.
As opposed to present amplitude/phase modulation amplifiers, the
signal of the present invention is not first produced then
amplified. Such a method would limit the efficiency of the
amplifier to those of linear amplifiers. The present invention
allows fixed amplitude signals to be amplified using high
efficiency amplifiers. The amplified signals are then combined
using a combiner. This combiner allows the information contained in
the phase of one of the signals to be transfered to the amplitude,
hence producing a high efficiency amplitude/phase modulated signal.
It can be seen that the operation of an amplitude/phase modulated
amplifier no longer has to depend on low efficiency AM
amplifiers.
Referring to FIG. 5 a block diagram of a communication device 300
in accordance with the present invention is shown. It is understood
that the only elements shown here are those that are necessary to
describe the principles of the present invention. The communication
device 300 is prefereably a transmitter used for the transmission
of QAM signals. A modulator 310 receives voice from a microphone
308. Keyboard information is coupled to the modulator 310 from a
keyboard. The signals from the microphone 308 and the keyboard 304
are processed and used to phase modulate a carrier signal using
well known phase modulation techniques. The two phase modulated
signals 110 and 112 are coupled to the amplifier 100 where they are
used to produce a QAM signal as described above. The output of the
amplifier 100 is subsequently coupled to the antenna 108. A
controller 306 is used to control the operation of the modulator
310 and other components of the device 300.
In summary, an amplitude/phase modulated signal is produced by
first phase modulating the carrier signal with the two modulating
signals to produce two distinct phase modulated signals having the
same amplitude. These signals are individually amplified using
non-linear switching amplifiers. These amplifiers can be used
because the information is not yet contained in the amplitude. The
two amplified signals are then combined in a combiner to produce
the amplitude/phase modulated signal. The combiner shifts the
information contained in the phase of one of the signals to the
amplitude of the resultant signal. Using this method to produce an
amplitude/phase modulated signal allows one to take full benefit of
amplitude/phase modulated signals without the deficiencies of AM
amplifiers.
* * * * *